1. Field of the Invention
The present invention relates to optical communications systems, terminals for use in systems, to optical links including such terminals, to cellular and radio distribution points and base stations and in particular, but not exclusively, to radio and microwave systems, including cellular radio systems, including such links. 2. Related Art
It is known that one of the principal problems that must be addressed before optical fibre networks can be extended from trunk networks to the local loop is the cost and complexity of the remote terminals. These terminals need to be able to both receive and transmit optical signals. Most solutions to this problem use lasers as optical transmitters but this necessitates the use of control circuitry which increases the cost, complexity and electrical power consumption of the terminals.
A similar problem, although with a slightly different application, exists for optical fibre feeds to antennas for cellular and other radio and microwave systems where the cost and power consumption of terminal equipment used to supply signals to remote antennas are important factors. xe2x80x9cAntenna remotingxe2x80x9d as it is known is of particular interest for cellular and satellite systems. The frequencies involved are typically in the hundreds of Mhz to tens of GHz range. There is also interest in yet higher frquencies, for example for radar. While, strictly speaking, this range extends into what is more properly known as the millimeter wave band, throughout this specification the expression xe2x80x9cradio or microwavexe2x80x9d has been used loosely to cover this wider range. Thus, unless the context clearly requires otherwise, the term should be read in a broad way to include the millimeter wave band.
Embodiments of the present invention seek to solve, at least in part, these problems.
In the paper by Frigo et al, xe2x80x9cA wavelength division multiplexed passive network with cost-shared componentsxe2x80x9d, IEEE Photonics Technology Letters, 1994, volume 6, pp. 1365-1367, it is proposed that subscriber terminals in a passive optical network (PON) each be provided with an optical modulator in place of the conventionally proposed laser. A single, cost-shared tunable laser is provided in the central office which feeds the PON and this is wavelength stepped through the different wavelengths of the various optical network units (ONUs) which feed the subscribers connected to the PON. In each subscriber""s ONU, the laser light received from the shared laser is split by a passive tap, with a portion of the light being detected by a receiver. The remainder is xe2x80x9clooped-backxe2x80x9d towards the central office through the modulator. The relevant subscriber uses the modulator to modulate the time-slice of light received from the shared laser. The up-and-down-stream signals can be separated at the central office by time partitioning, wavelength, sub-carrier frequency, format, modulation depth, gating, coding, etc. The preferred separation technique involves the use of radio frequency sub-carrier modulating the downstream (from the central office) data. As described, half of the packet of light in each ONU""s laser timeslot is modulated at the central office, the other half is modulated at the ONU to provide the upstream data link. The same RF sub-carrier frequency is used for the up and down stream signals. Use of the technique for the transmission of video on demand signals is described. The nature of the modulator used in the ONU is not revealed.
Wood et al describe, in xe2x80x9cBidirectional fibre-optical transmission using a multiple-quantum-well (MQW) modulator/detectorxe2x80x9d, Electronics Letters, 1986, volume 22, pp. 528-529, a bidirectional optical fibre transmission system in which one end of the fibre link has an MQW modulator in place of the more conventional laser and detector pair. At the other end of the single-fibre was a laser, and a beam-splitter was used to divert the return optical signal to an avalanche photodiode receiver. To send data from the MQW modulator to the laser site, the laser was operated quasi-CW and the modulator, which included a gold mirror and was operated in reflection mode, used to intensity modulate the reflected light. In the opposite direction the laser was directly modulated and the MQW modulator was used as a photodetector. While no changes were made to the optical system to achieve reversal of the direction of information flow, it was necessary both to re-arrange the electrical drive components and to modify the DC biases of the laser and of the MQW device. It was thus not possible to have a fully duplex operation (i.e. simultaneous transmission in both directions). Half-duplex operation would have required some electrical switching and bias adjustment function to control the bias level and to effect the re-arrangement of the circuit in synchronism with the half duplex rate. In fact, no such electrical control arrangement is suggested in the paper.
R B Welstand, et al, describe in xe2x80x9cDual-Function Electroabsorption Waveguide Modulator/Detector for Optoelectronic Transceiver Applicationsxe2x80x9d, IEEE Photon, Tech. Lett. Vol.8, No. 11, pp 1540-1542, a bulk (non-MQW) electroabsorption modulator device which is useful both as a modulator and as a detector. The device is referred to as an optoelectronic transceiver. A suggested application of the device is in antenna remoting. Again, the transceiver requires an adjustable dc electrical bias to switch from modulator to detector operation. It is explained that the transmit mode and the receive mode can be remotely switched by control circuitry which can adjust the dc electrical bias with a switching time limited by the associated electronics. Separate experiments were performed to determine the optimum performance of the device as a modulator and as a photodetector. In the modulator assessment, different bias levels were used depending upon the type of performance required. Optimum bias levels of 2.0 and 2.93 volts were found. It was also explained that, in order to maintain high suboctave and multioctave spurious-free dynamic range modulator performance over temperature, active modulator bias control was required. In the detector experiments, the device was biased at 7.0 volts. There is no hint or suggestion that the device could provide both funtions simultaneously. Thus, again it is clear that full duplex operation was not possible.
The present inventors have discovered that it is possible to provide good performance over both transmission directions in an optical link in which an optical modulator is used both as a modulator and as a detector even when the modulator is subject to the same DC operating conditions in both operational modes. That is, as distinct from the above-identified teachings, systems according to the present invention do not need complex electrical bias control. Indeed, for very many commonly available modulators, perfectly acceptable system performance can be achieved with no bias at all, that is with zero bias.
The fact that a single bias level can be used for transmission in both directions makes possible full duplex operationxe2x80x94that is, simultaneous transmission in both directions. Of course, the invention also has application to systems which are not, or are not run, full duplex. The advantages of simplicity, compactness and low-cost of terminal equipment, which all follow from the invention, are all equally useful in systems run half-duplex, e.g. ping-pong (time-division multiplex) systems. Again, for the many installations, which can be run without electrical bias in the remote terminal, the fact that no local power supply is needed is a tremendous benefit. This is especially true in the field of antenna remoting.
Accordingly, in a first aspect the present invention provides a method of communicating between a first node and one or more further nodes in an optical communications system, the method comprising;
i) receiving at the first node, an optical signal transmitted from a first further node over an optical fibre link;
ii) detecting, at the first node, the optical signal using an electro-optic device and producing thereby an electrical signal representing information carried by the optical signal; and
iii) imposing, using said electro-optic device, an information-carrying modulation onto the received optical signal and feeding the thus modulated optical signal into an optical fibre link to transmit it to a second further node; wherein step ii and step iii are carried out with the electro-optic device subject to the same DC operating conditions. Preferably said first further node and said second further node are a single node. Preferably the DC bias voltage applied is zeroxe2x80x94that is no DC bias at all is applied. Preferably, the electrical signal is an RF or microwave signal.
According to a second aspect of the invention there is provided an antenna installation for a radio or microwave transmission system, the installation comprising: antenna means for transmission and reception of radio-frequency or microwave signals; optical input and optical output ports; an optical modulator arranged in the optical path between, the optical input port and the output port and operatively connected to the antenna means and arranged in use to receive RF or microwave signals therefrom and to impose a corresponding modulating onto light received via said input port and which passes to said output port; and optical detection means arranged in use to receive optical signals via said input port and operatively connected to the antenna means so that RF or microwave modulation present in said optical signals is coupled out to said antenna means from which it is then radiated.
Preferably the optical detector means are provided by the optical modulator.
Preferably the optical detector means and the optical modulator are electrically unbiased in operation.
Additionally it is preferred that the modulator is an electro-absorption modulator.
Further preferably the terminal is sited remotely from a base station or central office from which it receives optical signals carrying an RF or microwave component.
According to a third aspect of the invention there is provided an optical communications network featuring a remote terminal characterised in that said terminal comprises an electro-optic device that
(i) detects an incoming optical signal from a first portion of the optical communications network whilst simultaneously
(ii) modulating said incoming optical signal, said optical signal being transmitted to a further portion of the optical communications network.
Preferably the optical signals are analogue signals. In addition it is preferred that the optical communications network uses a frequency division multiplexing system.
According to a fourth aspect of the invention there is provided a method of operating an optical communications network, the method being characterised in that an optical signal is respectively received and modulated simultaneously by controlling a single electro-optic device.